Printers, especially printers used to produce continuous tone or photographic images, require routine calibration to compensate for the use of different print media lots and types or the use of different inks, toners, donor ribbons, types and lots. Calibration also addresses printer electronics and components whose operating characteristics drift over time due to wear and usage.
Performing tone scale calibration for a printer requires that a plurality of printed patches on a target be measured by some means. The measurements are then processed through a calibration algorithm, which generates new printing parameters, such as a lookup table (LUT), to optimize the printed output. These measurements are usually made by an instrument that measures the reflective density, such as a densitometer or a spectrophotometer. Typically, the units of measurement are Status A density, which is a measure of the amount and or combinations of dyes or pigments present in a given patch. The calibration instrument's density measurement range is typically greater than the printer's own Dmin to Dmax density range. This greater range is desirable and required for most existing calibration methods, as the calibration instrument's measurements can be used to accurately and optimally calibrate the printer through its entire Dmin to Dmax density range.
However, there are several drawbacks to using these instruments for printer calibration. First, densitometers and spectrophotometers are expensive. They also require calibration themselves, require knowledgeable users and are ancillary equipment not used to produce prints or to scan hardcopy media digitalization or duplication. Lastly, densitometers and spectrophotometers use factory provided calibration targets, which are also expensive, and can be lost, damaged or degraded if they are improperly handled or stored. It is therefore desirable to be able to effectively use a less costly measurement device for printer calibration.
Reflective scanners, such as a flat-bed print scanner, can be utilized for this purpose and are readily available. However, these devices typically have a density measurement range that is smaller than that of the printer's output range and are not designed to produce a stable, invariant response across their entire response range. Reflective scanners measurements drift due to changes in lamp output, changes in electrical components, debris such as pollen and dust and film caused by off-gassing from plastic components within the scanner housing that collects on the underside of the scanner platen glass. In addition to variations due to drift over time and usage, scanners of this type vary between manufacturers and within productions lots.
It is known to use reflection scanners as input sources for printer calibration; however these techniques all have requirements that limit their accuracy and applicability.
U.S. Pat. No. 8,203,768 teaches a calibration method that includes scanning a test patch, which comprises a plurality of halftone cells, to obtain reflectance values, calculating subset averages of reflectance values as defined by an averaging window, and calculating an overall average based on the subset averages. This calculation pertains exclusively to halftone printing systems and integrates the halftone dot patterns to generate a reflectance value. The densitometers or spectrophotometers used in traditional printer calibration include an aperture that is typically around 5 mm in diameter, and the reflected light that passes through that aperture is optically averaged by the device. Reflection scanner based printer calibration for halftone images involves averaging values in some region analogous to the aperture of a spectrophotometer or densitometer.
U.S. Pat. No. 7,719,716 describes techniques for using a scanner to calibrate printers and requires that reflectance value be calculated for each patch on a test target. This method would preclude using test targets with patches that are within the gamut of the printer, but outside the accurate gamut of the scanner.
U.S. Pat. No. 7,319,545 assumes the scanner is a relatively stable measurement device and will remain in a state that is sufficiently close to its intended design such that it does not need to be characterized. The disclosure assumes that the drift associated with the printer will be much greater than the variability associated with the scanner. However, in reality, density measurement deviations for reflective scanners can be large in certain density regions, especially on worst case scanner types.
U.S. Pat. No. 6,909,814 describes converting data from an object scanner and then correcting that data whenever the object scanner response does not correspond to that of a standard scanner response. Every object scanner must be so characterized. Every object scanner must have a reference to compare it to the results of a standard scanner and this scanner calibration has to be done from time to time. It is impractical to correct every scanner on a routine basis.
U.S. Pat. No. 6,671,067 requires that a factory produced reference target and a printed target be scanned simultaneously, referred to as a “combined target.” As previously discussed, factory provided calibration targets are expensive, can be lost, and can fade or be damaged if improperly handled or stored.